The present invention relates to a silane cross-linked, chlorine-resistant polyolefin tube made by the so-called single-stage silane process.
Plastic tubular conduits are made from a large number of polymer materials.
In the field of materials for thermoplastic tubes particularly polyvinyl chloride, polypropylene, polyethylene, polybutylene or cross-linked polyolefin are utilized.
The cross-linking of polyolefins may be effected chemically or physically. As described, for example, in Kautschuk, Gummi, Kunststoff, 34th year, No. 3/1981, pages 197 ff, in the technically significant cross-linking technologies a distinction is made between radiation cross-linking, peroxide cross-linking and silane cross-linking.
The last-named process distinguishes itself from the other cross-linking methods primarily by the process technique: In a first process step the polymer chain radicals are generated with the aid of the usual radical initiators, to which, in a second process step, the silane molecules with their vinyl function are added. Such silane-grafted polymers may still be thermoplastically processed. The cross-linking proper takes places after shaping by a silane condensation reaction in the presence of heat and moisture. Such a so-called two-stage silane process is described in U.S. Pat. No. 3,646,155.
In contrast, British Patent No. 1,526,398 describes the so-called single-stage silane process. In this process all additives are simultaneously dosed with the polymer in a specially designed extruder for producing online the desired extruded material. Subsequently, cross-linking is effected in the presence of heat and moisture.
An application of a single-stage silane process, particularly for drinking-water tubes in the USA, is described in U.S. Pat. No. 6,284,178. In this process a residual methanol content of less than 12.2 ppm in the tube is obtained by using a maximum of 1.8 weight percent mixture of silane/peroxide/catalyst and by setting the duration of cross-linking at more than 4 hours. No mention is made concerning a chlorine-resistant provision of such a silane cross-linked polyethylene tube with special stabilizers. Rather, the combination of Irganox B215 and Irganox 1010 described in U.S. Pat. No. 6,284,178 has—because of the low melting point of the phenolic constituents—a much too low extraction resistance against chlorine water.
This prevents in practice the use of silane tubes made in accordance with U.S. Pat. No. 6,284,178.
The reason is that drinking water in the USA is provided, for purposes of disinfection, with a larger chlorine dose as compared to European conditions. As known by the specialist, at an appropriate pH-value, chlorine water may produce hypochlorous acid HOCl which is strongly oxidizing and therefore may lead to a premature failure of the tube.
It is the object of the invention to provide a silane cross-linked polyolefin tube which is made in a one-stage process and which is chlorine-resistant at a chlorine content between 0.1 and 5 ppm, which has minimum degree of cross-linking of 60%, and which further satisfies the standard specifications for cross-linked polyethylene tubes, set by the various ASTM and NSF norms.